This invention relates to a method for producing an image of a submerged object, e.g. a shipwreck or the sea bottom.
Acoustic sensors have become increasingly more common in systems for underwater sensing and imaging. Sonars are often used, ranging from simple systems detecting echos of an emitted pulse, to side scan sonars or two-dimensional multibeam sonar systems emitting and receiving signals along a line within a chosen angle and three-dimensional acoustic cameras, such as described in the articles xe2x80x9c3D ACOUSTIC CAMERA FOR UNDERWATER IMAGINGxe2x80x9d by Rolf Kahrs Hansen and Poul Arndt Andersen in Acoustical Imaging, vol. 20, Plenum Press, New York, 1993, and xe2x80x9cA 3D UNDERWATER ACOUSTIC CAMERA-PROPERTIES AND APPLICATIONSxe2x80x9d by R. K. Hansen and P. A. Andersen in Acoustical Imaging, vol. 22, Plenum Press, New York, 1996.
In imaging larger objects the two-dimensional sonar is normally used by moving the sonar over e.g. the sea bottom and scanning at an angle essentially perpendicular to the direction of the movement. The data sampled along each line are combined to provide a segmented picture of the sea floor. A problem inherent in this solution is the difficulties in controlling the exact position of the sensor. A sonar unit being moved over the sea bottom is subject to drift because of current, wind, and if the sonar is carried by a boat, and inaccuracies in the control system of the vessel.
It is an object of this invention to provide a method for generating an image of the sea floor or under water objects using a 3D acoustic imaging system.
It is also an object of this invention to provide a method for generating a composite image of underwater structures being compensated for variations in the position or the movement of the acoustic transducers.
These objects of the invention are obtained using a method according to claim 1.
The advantageous features of the invention related to the use of a 3D acoustic transducer system are thus that each 3D segment contains data being virtually insensitive to movements of the recording transducer unit, that the 3D segments have been combined in order to provide a larger 3D image.
Another advantageous feature is that the 3D segments have a coordinate accuracy which is better than the accuracy of the position measurement system, due to corrections based upon the information content in the separate 3D image elements.
Yet another advantageous feature of this invention is that each underwater object, due to the overlapping images, are insonified several times, from different angles of incidence. If one angle of incidence does not cause reflected energy to be transmitted back to the transducer unit, the next angle of incidence might, thus providing a 3D image of the objects to a certain degree comprising views if the inside or back side of the objects. Therefore, much more detail will be available in the combined image than in one separate 3D image, or images composed of measurements from side scan sonars.